This invention relates to the transportation of perishable products such as vegetables, fruit, and other agricultural and industrial products which must be maintained in a controlled atmosphere if they are to arrive in good condition at a distant destination. Usually it is necessary or desirable to cool or even freeze such products but in certain cases it is necessary to warm them if they or their containers are exposed en route to an excessively cold ambient atmosphere.
It is also known that certain fresh products are best stored in a modified atmosphere--that is, not the normal mixture of gases and vapour in the ambient atmosphere, but a special mixture which may contain preservatives and other substances in proportions best suited to the particular product. Much is already known about the modified atmosphere best suited to many varieties of the food products transported in bulk about the world, and the application of modified atmosphere technology is now widely practised in several continents. Naturally it is necessary, if a modified atmosphere is applied, to seal the container in which the products and the gas mixture are housed. for certain perishable products it is possible that the modified atmosphere includes or comprises a liquid.
In the past decade, with the advent of containerisation, several large container ships particularly adapted for the transportation of perishable products have been designed and put into commission. A typical refrigerated container ship of this type plying between Europe and Southern Africa can carry a total of 2 762 TEU (twenty-foot equivalent units) standard containers, of which 712 so-called fruit containers capable of being chilled and 176 containers capable of being frozen are carried below deck. At the data of filing this application seven container ships of approximately similar capacity are in operation between Europe and Southern Africa, and other such ships in other routes. A full description of one such ship is contained in an article entitled "Containerschiff `Transvaal`", published in the German periodical, Hansa, volume 18 (1978), 1473-1485.
Each of these ships is equipped with a cold air refrigeration system serving the insulated holds in which the containers (themselves insulated) are stacked. A typical hold accommodates 360 containers, packed in nine-high stacks, twenty such stacks extending in a row thwartship, and two such rows of stacks making up the hold. Each stack of nine containers is known as a slot.
Each slot is served by a refrigeration column located alongside the central zone of one of the vertically aligned ends of the containers, and contains its own cooling coils and ducting for the supply of cold air and the withdrawal of air once it has passed through the containers. The temperature at which the cooling coil operates can be regulated individually for each refrigeration column so that different products, requiring differing degrees of cooling, can be accommodated in different slots; but each container in the same slot is subjected to cooling by air at the same temperature.
The containers used below decks hitherto have been sealed and insulated to prevent heat losses and, to allow the circulation of cold air, have been formed in the end facing the refrigeration column with two apertures, one near the upper edge and the other near the lower edge. The refrigeration column is fitted with a series of inflatable rings and, when the container is in position and the rings and inflated, each ring surrounds an aperture in the container wall. The rings are inflated once the slot is occupied, and provide a seal between the mouth of the apertures and a corresponding aperture in the ducting of the regrigeration column.
Hitherto it has not been feasible to apply modified atmosphere to the containers transported in the holds of such ships since the cooling of the containers has called for the closed-cycle circulation of cold air from the ship's refrigeration system. One of the consequences has been that the constant exposure of the products packed in the containers to the cold air circulating through them has dehydrated the products substantially on the relatively long voyages undertaken by the ships, so that the products do not arrive in their best condition, and certain products particularly susceptible to dehydration have not been transportable at all.
Containers are known which incorporate, in one of the container bodies, a refrigeration unit which is operated by external electrical power supplied by a cable which is plugged into a terminal in the container itself. The container is sealed and insulated and it is possible for it to contain a modified atmosphere which is cooled by the refrigeration unit. The refrigeration unit is self-contained and the container therefore discharges to the atmosphere the heat generated by the operation of the unit. These containers cannot be used in the hold of a refrigerated container ship of the kind described above since the heat generated by them would be discharged into the hold and would raise the temperature to unacceptable levels. Moreover, the individual refrigeration units, which require relatively frequent maintenance, would be largely inaccessible when the containers are stacked in the hold.